I don't think you will ever have a problem mixing developers as you suggest, i.e., making a stock solution A with everything minus the activator(s) and a stock solution B with the alkali (carbonate, metaborate, etc., etc.) and then combining the two just before developing. Many developers are formulated to be used exactly this way.

Those that aren't aren't adversely affected by being mixed fresh just before use (except the possibility of undissolved particles...). The real question, I suppose, is whether dividing the stock solutions will get you any real increase in shelf life. That would have to be tested. As for eliminating the slight increase in activity of D-76 when it ages, this should do the job splendidly.